I. Field of the Invention
This invention relates to controlling the pH of an aqueous liquid reservoir system such as would be found in the typical operation of cooling towers which circulate water through process heat exchangers for indirect heat transfer. Likewise the invention relates to any similar liquid reservoir system in which pH is subject to consistently increasing or consistently decreasing an inherent characteristic of the liquid reservoir system where it is desireable to maintain the pH within certain limits. The pH control system also relates to sequentially or intermittently injecting pH affecting liquid into the reservoir for maintaining the desired pH in the aqueous liquid system reservoir. It should also be noted that the inventive system relates to maintenance of basic and acidic aqueous liquid systems pH control.
II. Description of Prior Art
In the past much effort has been made in the area of understanding the mechanism and techniques of controlling pH in an aqueous liquid system. In all such systems chemical reactions would be taking place in varying degrees. As well pointed out in the prior art, pH is a non-linear function in as much as it represents a logarithmic function of the concentration of the hydrogen ion. Thus pH control, where the pH is determined and then an acidic or basic liquid is induced into the system to raise or lower the pH to the desired level, is more complex than a straight forward linear control of other reactants in a chemical process.
U.S. Pat. No. 4,053,743 to Niemi, Method for Controlling the pH and Other Concentration Variables, as well as U.S. Pat. No. 4,239,493 to Niemi et al, Method an Apparatus for Controlling the pH, amply discussed the problems associated with controlling the pH in chemical reaction systems. The Niemi et al patent describes a system in which the pH measurement is transformed to a concentration quantity such as hydrogen ion concentration and the reference value of pH is likewise transformed into the same concentration quantities such as hydrogen ion concentration. Then the patentees determine by means of a disassociation equation, a final feedback quantity which may be a function of independent concentration quantity based on other measurements at the output of the system in addition to the function of the concentration suggested as the hydrogen ion concentration. The patentees then suggest determination of a final reference value which may depend upon the independent concentration quantity as well as the concentration suggested as the hydrogen ion concentration.
In the chemical process industry water is used for indirect heat exchange and is recirculated through a cooling tower arrangement where it is cooled and collected for recirculation in the basin or tower bottom. In such a system the water entering the tower is cooled by air from outside as water drops from the place of entry through the tower to the basin in the bottom of the tower. During the process some of the water evaporates and some is carried from the tower entrained in the air.
A result of this water loss is to increase the concentration of dissolved solids or salts in the cooling tower water system. To prevent an excessive increase in salt concentration, water from the system is discharged, the operation is blown down, and evaporative and entrainment water losses or blow down losses are replaced by fresh or makeup water.
In order to prevent precipitation of ions in solution thus forming scale on the surface of the system or reducing the efficiency of heat exchange the pH of the water must be lowered by the addition of an acid. In aqueous systems problems arise in controlling pH and salt concentration because of wide fluctuations in pH and other characteristics such as conductivity.
In these situations, heat exchangers and cooling tower treating has previously been dealt with in a manner in which the pH of the treated water in the cooling tower basin is determined from a side stream off of the output from the cooling tower basin to the heat exchange system. In the case where the pH is determined to be above the desired range a pH controller coupled to a pH cell would activate a timer unit or control system to open valves and discharge a measured quantity of acid directly into the cooling tower basin. One such system is described in U.S. Pat. No. 787,281 to J. C. Ward, Jr., as assignor to Phillips Petroleum Company. The Phillips patent describes a system using air pressure and pneumatic controls responsive to the pH controller output for controlling the addition of acid to the cooling tower basin.
One of the problems in such a system is well pointed out in the Phillips patent which states that although pH of water in the cooling tower basin is normally controlled within a desired range by the intermittent introduction therein of a predetermined volume of acid from the acid source the rate of change of pH of water circulating through the heat exchanger system will at times vary to an extent that the measured amount of acid normally admitted to the cooling tower basin is more than necessary for regulating of the pH of the water within the desired range, and consequently the pH falls to a value below the intended range which is undesired from the standpoint of unduly high corrosive effects of the low pH water. The Phillips patent describes that the excessive introduction of acid to the cooling tower basin is reduced by continuous measuring pH of water in basin by circulating a side stream from the discharge line through the pH cell, then when the pH controller detects the pH below the predetermined minimum it operates to prevent additional acid from entering the cooling tower basin until the system itself has recovered.
However, one of the deficiencies in the Phillips system, and in the cooling tower art in general, is if the pH cell gave a false signal which indicated demand for acid within the desired range acid would continually be placed into the coolinq tower basin without triggering the pH controller to stop the pressure system for adding additional acid. This clearly could happen if the side stream became clogged and the pH cell continuously read a stagnant liquid side stream.
Other patents obtained by Applicants' patent search are as follows:
______________________________________ U.S. Pat. No. Date of Issue ______________________________________ 3,733,473 May 15, 1973 4,762,796 August 9, 1988 4,273,146 June 16, 1981 ______________________________________